Alternative digits implementation

…onnection-logic (#2) (dashpay#1558)

* Move MarkBlockAsReceived out of ProcessNewMessage

* Remove network state wipe from UnloadBlockIndex.

UnloadBlockIndex is only used during init if we end up reindexing
to clear our block state so that we can start over. However, at
that time no connections have been brought up as CConnman hasn't
been started yet, so all of the network processing state logic is
empty when its called.

Additionally, the initialization of the recentRejects set is moved
to InitPeerLogic.

* Move all calls to CheckBlockIndex out of net-processing logic

This will result in many more calls to CheckBlockIndex when
connecting a list of headers (eg in ::HEADERS messages processing)
but its only enabled in debug mode, and that should mostly just be
during IBD, so it should be OK.

* Move FlushStateToDisk call out of ProcessMessages::TX into ATMP

* Move nTimeBestReceived updating into net processing code